TOKYO: Thousands of farmers rallied in Tokyo on Wednesday against an Pacific-wide free trade pact as Prime Minister Yoshihiko Noda's prepares for a visit to the United States.

Noda had reportedly been looking to announce Japan would become a full-fledged participant in talks on framing the Trans-Pacific Partnership (TPP) during his trip to Washington next week.

But fierce opposition, including from within his own party, means he is unlikely to be able to give Barack Obama the good news on a pact the US president has pushed, partially as a way to counterbalance China's growing economic might.

Government spokesman Osamu Fujimura said Wednesday any announcement on Japan's participation in talks aimed at abolishing tariffs and other barriers was "premature".

Japan's Central Union of Agricultural Cooperatives (Zenchu) has campaigned vigorously against participation, saying a deal would reduce food security in a country where farmers -- especially of rice -- enjoy generous government protection, paid for by sky-high consumer prices.

"Abolishing tariffs without exceptions and reviewing regulations means that the pact forces us to fight unarmed against the United States and Australia, where farms are huge," said Akira Banzai, the chairman of Zenchu.

"If we accept more liberalisation (of trade in farm products), we will be badly damaged," Bandai told an applauding crowd of farmers from all over the country who had gathered in a park in the centre of the capital.

"The TPP is not a minor issue that can be treated as a present" to Obama on Noda's trip to the United States, he said.

Major businesses, academics and mainstream media have long pushed Tokyo to join the deal, which they say will improve access to foreign markets and boost regional trade and investment.

Australia, Brunei, Chile, Malaysia, New Zealand, Peru, Singapore, the United States and Vietnam are already signed up to take part in negotiations towards the deal.

Noda's November announcement that Japan would dip its toe in the talks sparked a rush of interest in what could become the biggest free trade zone in the world, accounting for 40 percent of global economic output.

The pact is a key plank of Washington's bid to engage with the fast-growing Asia-Pacific region, and the participation of Japan, the world's third largest economy, is seen as vital to its credibility.

Japan's exporters struggling under the weight of a stubbornly strong yen are eager for Tokyo to sign up, hoping greater access to foreign markets will help make up for the shrinking and greying population at home.

But the tens of thousands of ageing farmers who make up the backbone of Japan's countryside and form a powerful voting block are largely opposed, and say the TPP poses an existential threat to the country's way of life.

Kiyoko Tabata, 63, who runs a small rice farm in Wakayama, western Japan, said: "Protecting small-sized farmers means protecting communities in the countryside."